Couple tips to expand on Google and GunBroker:
When googling, add "forum" to your search terms. It'll limit your search to posts on boards like this, instead of the random crap trying to sell you something that often comes up.
On GunBroker, be sure to find the option for completed searches. This is great for less-common firearms because it gives you more examples. And it also avoids the problem of people throwing items up with ridiculous prices to see if anyone will bite. The market value is what someone will pay after all. I think you might have to verify a credit card with a $1 charge to be able to access this feature, but not certain.